In a multiple wavelength transmission system, a plurality of semiconductor laser elements having different band gaps are required. A semiconductor laser element is fabricated by forming, on a semiconductor substrate, a multiple quantum well active layer corresponding to the desired band gap wavelength. This multiple quantum well active layer is formed through crystal growth of a compound semiconductor on the substrate with the metal organic chemical vapor deposition (hereinafter described as MOCVD) method.
Therefore, for example, the Japanese Patent Laid-open No. 226563/1995 and Japanese Patent Laid-open No. 117040/1998 describe that a plurality of pairs of striped insulation film mask including interval of constant width are formed to the position where an optical waveguide path is formed and an optical waveguide path active layers of different film thickness and composition are formed thereto by changing the width of this stripe. Moreover, for example, the Japanese Patent Laid-open No. 100728/2000 describes that a plurality of active layers are formed by radiating heat ray having intensity distribution to a substrate when an active layer is formed with the MOCVD method.
However, the fabrication method described above is required, in order to obtain laser elements having different band gap wavelengths, to fabricate an element by preparing for a wafer where composition of active layer is different for every specification of band gap wavelength and therefore this method results in increase of fabrication cost.